Generally stated, hearing aids include a microphone for transducing detected sound, an amplifier for amplifying the electronic signals received from the microphone, and an earphone for transducing the amplified electronic signals into sound for hearing by the hearing aid wearer. The microphones and/or earphones used in such hearing aids often do not have a flat frequency response, but, rather, have a generally flat frequency response with an undamped peak across a known frequency range.
Feedback is a potential problem in such hearing aids since the output of the hearing aid must of necessity be much greater than the input and since there is often leakage of sound from the interior of the ear to the exterior of the ear proximate the microphone input. The feedback problem is exacerbated by an undamped response peak of the microphone which represents a very-high-gain condition over a narrow frequency range. In many cases, the overall gain of the hearing aid is purposely reduced at most frequencies so that the gain at the frequency of an undamped peak will not produce feedback.
A reduction in quality of delivered sound typically accompanies an undamped peak. An undamped peak can also result in user discomfort where complex sounds have an energy concentration in the vicinity of the undamped peak. Such discomfort may be eliminated by reducing the overall gain of the amplifier. This approach, however, results in a loss of gain at the quiet sound level such that the hearing aid wearer does not receive the full benefit of the hearing aid amplification.
Acoustic damping has heretofore used mechanical dampers to smooth the frequency response of microphones and earphones ("receivers") in order to smooth the overall frequency response of the hearing aid. The smooth response improves the overall performance of the hearing aid and helps prevent feedback.
In U.S. Pat. No. 3,930,560, Carlson and Mostardo described a fused-mesh mechanical damper. The damper described in that patent was subsequently made available as Knowles Electronic's BF-series dampers in 330, 680, 1000, 15000, 2200, 3300, and 4700 (cgs acoustic) Ohm values. A 1979 application note titled "Smoothing the ITE Frequency Response," and available from Knowles Electronics (Itasca, Ill.), described a "model BF-1743" damped coupling assembly incorporating that damper and designed to be mounted in the eartip of In-The-Ear (ITE) hearing aids. That damped coupling assembly provided a smooth response for the hearing aid earphone and permitted replacement of the damper when it became clogged with earwax or when a different value of damping resistance was desired. With that damped coupling assembly, a smooth hearing aid frequency response out to 16 kHz was practical.
Although mechanical damping mechanisms provide an improvement in the frequency response and performance of the hearing aids in which they are employed, such damping mechanisms are generally expensive and, further, are not entirely practical for some ears (especially in hot climates) since the damper elements tend to clog with earwax sometimes after only a few days. It is therefore desirable to have an alternative to such mechanical dampers.